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1.
A sedimentary record of human disturbance from Lake Miragoane,Haiti   总被引：1，自引：0，他引：1  
Lake Miragoane, Haiti is one of the largest, natural freshwater lakes in the Caribbean (A=7.06 km2, zmax=41.0 m, conductivity = 350 S cm–1). Lake waters are dominated by calcium and bicarbonate ions. The lake was thermally stratified, and oxygen profiles were clinograde during summer visits in 1983 and 1985. A 72-cm mud-water interface core was taken near the center of the lake and dated with 210Pb. The local 210Pb fallout rate is low (0.09 pCi cm–2 yr–1), about 20% of the global average. Bulk sedimentation rates ranged from 0.008 to 0.030 g cm–2 yr–1 during the past 130 years (0–8 cm depth). Sediment geochemistry and pollen have been analyzed in the topmost 58 cm of the section. Tentative ages were assigned to the core by extrapolation of 210Pb dates. According to this preliminary chronology, the bottom part of the core (58–30 cm) records pre-Columbian sedimentation (1000–500 B.P.) and contains pollen evidence of intact, dry and mesic forest. Pre-Columbian deposits are rich in organic matter (x = 30%) and relatively poor in carbonates (x = 15% as CO2). The top 30 cm of the core preserve the record since European contact (500 B.P. to present). Pollen data reveal two episodes of deforestation following European arrival. Consequent soil erosion is documented by a decrease in organic matter content (x = 15%) and an increase in carbonates (x = 27% as CO2). Surficial sediments reflect the widespread deforestation and soil loss that characterize the watershed today.  相似文献   

2.
A decade of widespread increases in surface water concentrations of total organic carbon (TOC) in some regions has raised questions about longer term patterns in this important constituent of water chemistry. This study uses near-infrared spectroscopy (NIRS) to infer lake water TOC far beyond the decade or two of observational data generally available. An expanded calibration dataset of 140 lakes across Sweden covering a TOC gradient from 0.7 to 24.7 mg L−1 was used to establish a relationship between the NIRS signal from surface sediments (0–0.5 cm) and the TOC concentration of the water mass. Internal cross-validation of the model resulted in an R 2 of 0.72 with a root mean squared error of calibration (RMSECV) of 2.6 mg L−1. The TOC concentrations reconstructed from surface sediments in four Swedish lakes were typically within the range of concentrations observed in the monitoring data during the period represented by each sediment layer. TOC reconstructions from the full sediment cores of four lakes indicated that TOC concentrations were approximately twice as high a century ago.  相似文献   

3.
We used 210Pb-dated sediment cores from wetlands and Blue Cypress Lake, in the Upper St. Johns River Basin (USJRB), Florida, USA, to measure historical accumulation rates of bulk sediment, total carbon (C), total nitrogen (N), and total phosphorus (P). Marsh cores displayed similar stratigraphies with respect to physical properties and nutrient content. Wetland sediments typically contained > 900 mg organic matter (OM) g–1 dry mass, > 500 mg C g–1, and 30–40 mg N g–1. OM, C, and N concentrations were slightly lower in uppermost sediments of most cores, but displayed no strong stratigraphic trends. Total P concentrations were relatively low in bottommost deposits (0.01–0.11 mg g–1), but ranged from 0.38–2.67 mg g–1 in surface sediments. The mean sediment accretion rate in the marsh since ~ 1900, 0.33 ± 0.05 cm yr–1, was calculated from ten 210Pb-dated cores. All sites displayed increases in accumulation rates of bulk sediment, C, N, and P since the early part of the 20th century. These trends are attributed to recent hydrologic modifications in the basin combined with high nutrient loading from agricultural, residential, and urban sources.  相似文献   

4.
This study tests the hypothesis that Fourier-transform infrared spectroscopy (FTIRS) of lake sediments can be used to infer past changes in tree-line position and total organic carbon (TOC) content of lake water. A training set of 100 lakes from northern Sweden spanning a broad altitudinal and TOC gradient from 0.7 to 14.9 mg/l was used to assess whether vegetation zones and TOC can be modelled from FTIR spectra of surface sediments (0–1 cm) using principal component analysis (PCA) and partial least squares (PLS) regression. Preliminary results show that FTIRS of lake sediments can be used to reconstruct past changes in tree line and the TOC content of lake water, which is hardly surprising since FTIRS registers the properties of organic and minerogenic material derived from the water mass and the drainage area. The FTIRS model for TOC gives a root mean squared error (RMSECV) of calibration of 1.4 mg/l (10% of the gradient) assessed by internal cross-validation (CV) yielding an Rcv2 of 0.64. This should be compared with a near-infrared spectroscopy (NIRS) and diatom transfer function for TOC from the same set of lakes, which have a Rcv2 of 0.61 and 0.31, and RMSECV of 1.6 and 2.3 mg/l, respectively. The FTIRS-TOC model was applied to a Holocene sediment core from a tree-line lake and the results show similar trends as inferences from NIRS and pollen from the same core. Overall, the results indicate that changes in FTIR spectra from lake sediments reflect differences in catchment vegetation and TOC, and that FTIRS-models based on surface-sediment samples can be applied to sediment cores for retrospective analysis.  相似文献   

5.
Geochemical anomalies and stable isotope ratios (18O, 13C) in authigenic carbonates and organic matter (13C) from a 660-year sediment core from Lake Chenghai, southern China, provide a continuous history of recent lake eutrophication. The multi-proxy geochemical and isotopic record can be divided into a three-part history of contrasting limnological development, including: (1) a clear-water, oligotrophic open lake system (1340 and 1690 AD); (2) an environmentally unstable, hydrologically closed, oligotrophic lake system (1690–1940 AD); and (3) an increasingly eutrophic, closed lake system marked by higher organic matter, nitrogen, CaCO3, and pigment concentrations, and lower 18O and 13C values in authigenic calcite (1940–1999 AD). The unanticipated lowering of 18O and 13C of authigenic calcite during eutrophication is thought to be the result of disequilibrium water–carbonate fractionation of oxygen and carbon isotopes during periods of elevated primary production, pH, and [CO3 2–] activities in the water column. The recent eutrophication of Lake Chenghai indicated by these geochemical proxies is essentially simultaneous with large-scale human migration and the application of agricultural fertilizers in the catchment area during the 20th century.  相似文献   

6.
The morphology and geochemistry of saline lakes in the Wadi El Natrun depression were studied. All lakes had pH values of 8.5--9.5 and a salinity from 283 to 540 g/L. The main ionic components were sulphate, chloride, carbonate and sodium. Traces of magnesium were also present. The water of the lakes is of the Cl– to SO 4 2– -Cl– type. Increased Cl– in Wadi El Natrun brines can increase metal solubility due to the formation of soluble chloro-complexes of trace elements. The metal concentrations decrease in the order: Pb > Cu > Cd > Ni > Zn > Fe > Mn. The characteristics of Wadi El Natrun saline lakes are compared with those from other saline lakes.  相似文献   

7.
Paleoshoreline evidence for postglacial tilting in Southern Manitoba   总被引：1，自引：0，他引：1  
Detailed air photo interpretation and four seasons of field mapping and surveying in southern Manitoba have revealed that the once-level paleoshorelines of Lake Winnipegosis and Dauphin Lake and the Burnside shoreline of former Lake Agassiz have been tilted up to the northeast by postglacial differential rebound. Our investigation has also revealed that Lake Winnipegosis has the best preserved paleoshoreline record of any of the large lakes in southern Manitoba, including lakes Winnipeg and Manitoba. This is because northeasterly uptilting shifts the region's lakes to the southwest. Lakes with southern outlets, like Lake Winnipegosis, undergo general regression as the outlet is lowered relative to the rest of the basin. Lakes with northern outlets, like lakes Winnipeg and Manitoba, undergo general transgression as northeasterly uptilting raises the outlet relative to the rest of the basin. Along the northeastern shore of Lake Winnipegosis a staircase of at least 32 abandoned Winnipegosis shorelines exists that is consistent with northeasterly tilting. The Dawson level represents the major mid-Holocene highstand on Lake Winnipegosis. It persisted for about 500 years, peaking at 5290 14C yr B.P. (early Dawson) and then falling about 3 m by 4740 14C yr B.P. (late Dawson). The early Dawson shoreline is tilted at 13.5 cm km-1 in a direction N24.3°E. Three other shorelines informally named shoreline 4, shoreline 3, and shoreline 2 are also tilted up to the northeast. Their radiocarbon ages (and slopes in cm km-1) are 3330 yr B.P. (2.2), 1510 yr B.P. (1.3), and 1080 yr B.P. (0.7), respectively. On Dauphin Lake shoreline IV is the oldest level mapped for this study. It has a 14C age of 7910 yr B.P. and is tilted at 21.7 cm km-1 in a direction N44.4°E. The Id shoreline marks the major mid-Holocene highstand for Dauphin Lake. It peaked at 4640 14C yr B.P. followed by a rapid decline of about 1 m to the Ib shoreline, which is dated at 4320 14C yr B.P. Id is tilted up at 8.8 cm km-1 in a direction N53.4°E. The next major shoreline is Ia3 which has a 14C age of 3020 yr B.P. and is tilted up at 5.3 cm km-1 in a direction N62.3°E. Tilt directions are significantly more easterly for the Dauphin Lake shorelines than those from Lake Winnipegosis or any of the much older Lake Agassiz shorelines. Taken together, the Winnipegosis and Dauphin isobases indicate that the direction of tilt in southern Manitoba is more complex than a simple uni-directional pattern. The observed pattern of tilting for paleoshorelines in southern Manitoba agrees better with predictions derived from the recently revised loading history model ICE-4G than with those from its predecessor ICE-3G. In general, the calculated tilt based on the ICE-3G load tends to exceed the observed tilt, while ICE-4G tends to underestimate it. Both ice load models appear to disagree most with our observed tilts in this region during the interval before about 9000 cal yr B.P., when deglaciation was proceeding rapidly and the large water load associated with Lake Agassiz covered the region. Because both of these ice load models have been estimated mainly from a global data set of relative sea level curves from marine coast sites, it is not unexpected that model tilts derived from them do not agree well with observations in the North American continental interior. The pattern of postglacial crustal deformation for southern Manitoba described in this paper could be used to further refine ice load models for the North American continental interior.  相似文献   

8.
Geomorphology of a beach-ridge complex and adjacent lake basins along the northern shore of Lake Michigan records fluctuations in the level of Lake Michigan for the last 8000 to 10 000 14C yr B.P. (radiocarbon years Before Present). A storm berm at 204.7–206 m (671.6–675.9 ft) exposed in a sandpit provides evidence of a pre-Chippewa Low lake level that is correlated with dropping water levels of Glacial Lake Algonquin (c. 10 300–10 100 14C yr B.P.). Radiocarbon dates from organic material exposed in a river cutbank and basal sediments from Elbow Lake, Mackinac Co., Michigan, indicate a maximum age of a highstand of Lake Michigan at 6900 14C yr B.P., which reached as high as 196.7 m (645 ft), during the early-Nipissing transgression of Lake Michigan. Basal radiocarbon dates from beach swales and a second lake site (Beaverhouse Lake, Mackinac Co.) provide geomorphic evidence for a subsequent highstand which reached 192.6 m (632 ft) at 5390±70 14C yr B.P.Basal radiocarbon dates from a transect of sediment cores, along with tree-ring data, and General Land Office Surveyor notes of a shipwreck, c. A.D. 1846, reveal a late-Holocene rate for isostatic rebound of 22.6 cm/100 radiocarbon years (0.74 ft/100 radiocarbon years) for the northern shore of Lake Michigan, relative to the Lake Michigan-Lake Huron outlet at Port Huron, Michigan. Changes in sediment stratigraphy, inter-ridge distance, and sediment accumulation rates document a mid- to late-Holocene retreat of the shoreline due to isostatic rebound. This regression sequence was punctuated by brief, periodic highstands, resulting in progressive development over the past 5400 14C yr of 75 pairs of dune ridges and swales each formed over an interval of approximately 72 years. Times of lake-level fluctuation were identified at 3900, 3200, and 1000 14C yr B.P. based on changes in inter-ridge spacing, shifts in the course of Millecoquins River, and reorientation of beach-ridge lineation. Soil type, dune development, and selected pollen data provide supporting evidence for this chronology. Late-Holocene beach-ridge development and lake-level fluctuations are related to a retreat of the dominant Pacific airmass and the convergence of the Arctic and Tropical airmasses resulting in predominantly meridional rather than zonal air flow across the Great Lakes region.This is the 13th in a series of papers published in this special AMQUA issue. These papers were presented at the 1994 meeting of the American Quaternary Association held 19–22 June, 1994, at the University of Minnesota, Minneapolis, Minnesota, USA. Dr Linda C. K. Shane served as guest editor for these papers.  相似文献   

9.
This paper reports on hydrochemical features of diluted waters in the source areas and the brine end-members dominant in the playa of the Salinas Grandes Basin, Córdoba, Argentina. Special emphasis was placed on the study of the relationship between geomorphology and the resulting hydrochemical fractionation. Inflow is from springs and mountain streams which disappear before reaching the saline complex. The playa and intermittent saline lakes are mainly fed by groundwater flow and a few moderately saline and perennial springs. Conversely, ephemeral lakes are fed by atmospheric precipitation and groundwater, whereas small ponds are only fed by atmospheric precipitation. The absence of a clear linkage between geomorphological units and water types was evident in the source areas. Up to four types of water were recognized in a given geomorphological unit. From the sandflat downward towards the playa, the correspondence between geomorphological units and water types was clear, coinciding with a decreased hydrochemical heterogeneity. In this subenvironment, I have recognized two dominant types of water (SO 4 2– –Cl––HCO 3 – –Na+ and Cl––SO 4 2– –HCO 3 – –Na+), which can be considered the original members of the neutral brine in the playa (Cl––SO 4 2– –Na+ where Ca2++Mg2+ do not surpass 5 meq per cent), and an intermediate type in the mudflat (Cl––SO 4 2– –Na+ where the contribution of Ca2++Mg2+ reaches up to 15 meq per cent). It seems evident that in the zone between the source areas and the sandflat, hydrochemistry is governed by chemical weathering. In stream floodplains and in the distal alluvial plain, close to the saline complex, the increase in water types as well as the increase of HCO 3 – with respect to SO 4 2– , were explained by the mixture of aquifers controlled by the Salinas Grandes-Salinas de Ambargasta fracture. In the saline complex, the more concentrated end-members are the result of evaporation of the two more frequent water-types in the sandflat subenvironment, and salt dissolution of ancient evaporite deposit.  相似文献   

10.
Total organic carbon (TOC), total nitrogen (TN) and total phosphorus (TP) were determined in combination with stable isotope ratios of carbon and nitrogen (δ13COrg, δ15N) in a 63 cm sediment core from Longgan Lake, located in the middle reaches of the Yangtze River, China. These geochemical and isotopic records provide a continuous history of lake productivity and trophic state of Longgan Lake since 1890. Variations of δ13COrg, TOC, TN and TP indicate that primary productivity of Longgan Lake increased continuously during the last century and that the trophic state of the lake shifted from oligotrophic to mestrotrophic conditions accordingly. Anthropogenic sources of organic carbon (OC), nitrogen (N) and phosphorus (P) were distinguished from their natural background in the sediments using mass accumulation rates. Element mass accumulation rates suggested increased human activities in the lake’s catchment since 1950s, were especially the utilization of artificial fertilizers amplified the anthropogenic input of N and P into the lake. In the course of the improved availability of dissolved nutrients also primary productivity of Longgan Lake increased, resulting in an increase of the Suess-effect corrected organic carbon isotope ratios. δ15N of bulk sediments show a marked shift towards lower values around 1950 that has been attributed to the input of nitrogen from chemical fertilizers characterized by relatively depleted isotopic signatures into the lake.  相似文献   

11.
We examined the relationship between three key environmental variables (water depth, loss-on-ignition, and bottom-water temperature) and fossil chironomid distributions sampled from within-lake gradients in three small, moderately deep (18–35 m), maar lakes on St Michael Island, western Alaska. Site-specific (one lake, 29 samples) and local (three lakes, 87 samples) inference models for reconstructing water depth were developed using partial least squares regression and calibration. These models and a previously published regional model (136 lakes, one central-lake sample from each) are used to infer water depths from 78 fossil samples spanning the last ~30,000 14C years B.P. at Zagoskin Lake. Although the site-specific [r 2 boot = 0.90, root mean square error of prediction (RMSEP) = 1.76] and local (r boot2 = 0.68, RMSEP = 4.36) inference models have better performance statistics than the regional model, few clear trends among all three models exist in the lake-level reconstruction. We propose that multiple, within-lake sampling of gradients can be used to improve the performance statistics of water-depth transfer functions and ultimately reconstruct paleohydrology in regions known to exhibit large fluctuations in moisture balance through time given that: (1) adequate analogs are established and (2) taphonomic processes important to benthic invertebrate remains are more fully understood.  相似文献   

12.
Paleohydrology studies at Mathews Pond and Whitehead Lake in northern Maine revealed synchronous changes in lake levels from about 12,000 14C yrs BP to the present. We analyzed gross sediment structure, organic and carbonate content, mineral grain size, and macrofossils of six cores from each of the two lakes, and obtained 72 radiocarbon dates. Interpretation of this paleo-environmental data suggests that the late-glacial and Younger Dryas climate was dry, and lake levels were low. Early Holocene lake levels were considerably higher but declined for an interval from about 8000 to 7200 14C yrs BP. Sediment of both lakes contains evidence of a dry period at ∼7400 14C yrs BP (8200 cal yr). Lake levels of both sites declined abruptly about 4800 14C yrs BP and remained low until 3000 14C yrs BP. Modern lake levels were achieved only within the past 600 years. The west-to-east, time-transgressive nature of lake-level changes from several sites across northeastern North America suggests periodic changes in atmospheric circulation patterns as a driving force behind observed moisture balance changes. Electronic supplementary material to this article is available at and accessible for authorized users.  相似文献   

13.
Climate in central Asia is dominated by the Asian monsoon. The varying impact of the summer monsoon across the Tibetan (Qinghai-Xizang) Plateau provides a strong gradient in precipitation, resulting in lakes of different salinity. Diatoms have been shown to indicate changes in salinity. Thus, transfer functions for diatoms and salinity or related environmental variables represent an excellent tool for paleoclimatic reconstructions in the Tibetan Plateau. Forty freshwater to hypersaline lakes (salinity: 0.1 to 91.7 g l–1) were investigated in the eastern Tibetan Plateau. The relationship between 120 diatom taxa and conductivity, maximum water depth and major ions were analyzed using an indicator value approach, ordination and taxon response models. Canonical correspondence analysis indicated that conductivity was the most important variable, accounting for 10.8% of the variance in the diatom assemblages. In addition water depth and weathering were influential. Weighted Averaging (WA) and Weighted Averaging Partial Least Square (WA-PLS) regression and calibration models were used to establish diatom-conductivity and water depth transfer functions. An optimal two-component WA-PLS model provided a high jack-knifed coefficient of prediction for conductivity (r2 jack = 0.92), with a moderate root mean squared error of prediction (RMSEPjack = 0.22), a very low mean bias (0.0003), and a moderate maximum bias (0.26). A WA model with tolerance downweighting resulted in a slightly lower r2 jack (0.89) for water depth, with RMSEPjack= 0.26, mean bias = –0.0103 and maximum bias = 0.26.  相似文献   

14.
Diatom abundances in the surface sediment samples of 41 mountain lakes in the central Austrian Alps (Niedere Tauern) were related to environmental variables using multi-variate techniques. Canonical correspondence analysis (CCA) revealed that the pH, date of autumn mixing (A mix), mean August water temperature (T Aug), dissolved organic carbon (DOC), and relative water depth (Z rel) made significant contributions to explain the diatom assemblage variation in the lakes of the training set. A weighted averaging partial least square regression and calibration model was used to establish Di-pH (R 2 boot= 0.72, RMSEPboot= 0.131), and a thermistor measurements-based PLS model for A mix (R 2 boot= 0.71, RMSEPboot= 0.006 log10 Julian days). The latter showed a better prediction than T Aug, and was used in terms of climate change. These transfer functions, together with analyses of loss on ignition (LOI), the total carbon/nitrogen (C/N)-ratios, and selected pollen, were applied to an early to mid-Holocene (11.5–4 cal. ky BP) sediment core section from an Austrian Alpine treeline lake on crystalline bedrock. Additionally, passive sample scores in the CCA of the diatom training set were used to show trends in the variables DOC and Z rel. During the early Holocene, diatoms indicative of increased pH, extended warm summers, and low water levels dominated. Between 10.2 and 7.6 cal. ky BP it was followed by diatom assemblages that indicated an increase in lake water depth and an earlier A mix. The multi-proxy data suggest that the A mix decline is the result of a series of snow-rich summer cool and wet climate fluctuations, which were divided by climate warming at ∼9 cal. ky BP. Increased A mix, LOI and DOC, and the correspondent decline in the C/N-ratios, show subsequent climate warming between 7.3 and 6 cal. ky BP. The long-term trend in Di-pH indicates the impact of catchment-related processes during the early-Holocene, that were superimposed by climate.  相似文献   

15.
Stable carbon and nitrogen isotope ratios have been determined on 41 strains ofArtemia sp. from different geographic regions around the world. The δ13C and δ15N values ranged between −13.7 to−25.0 per mil and −0.7 to 21.2 per mil respectively.Artemia δ13C values from coastal environments are consistent with a marine origin for the food sourceArtemia from inland salt lakes have a range of carbon isotope values suggesting C3, C4 and CAM based organic matter could form the base of theArtemia food chain. These data indicate thatArtemia having a wide range of carbon and nitrogen isotope values are available for tropho-dynamic research studies that quantify the effect of respired CO2 on tissue and CaCO3 shell13C/12C ratios. Such stable isotope variation also suggests that stable isotope fingerprinting remains a viable technique for identifying specificArtemia collection sites.  相似文献   

16.
We designed, constructed, calibrated and field-tested a lightweight (30 kg), 4.2 m diameter, 16.4 m3 polyethylene-covered dome static chamber ecosystem gas exchange cuvette that can quantify ecosystem CO2 and water vapour fluxes as low as 0.1 μmol CO2 m−2 s−1 and 0.1 mmol H2O m−2 s−1 with little impact on environmental conditions. Fluxes measured in May 2001 in an intact Great Basin sagebrush ecosystem at midday were significantly higher than in an adjacent post-wildfire successional ecosystem, with observed ranges from –0.71 to 1.49 μmol CO2 m−2 s−1 for CO2 and from –0.09 to 0.53 mmol H2O m−2 s−1 for water vapour.  相似文献   

17.
The trophic status of lakes in New Zealand is, on average, low compared to more densely populated areas of the globe. Despite this, trends of eutrophication are currently widespread due to recent intensification in agriculture. In order to better identify baseline productivity and establish long-term trends in lake trophic status, diatom-based transfer functions for productivity-related parameters were developed. Water quality data and surface sediment diatom assemblages from 53 lakes across the North and South Islands of New Zealand were analysed to determine species responses to the principal environmental gradients in the data set. Repeat sampling of water chemistry over a 12-month period enabled examination of species responses to annual means as well as means calculated for stratified and mixed periods. Variables found to be most strongly correlated with diatom species distributions were chlorophyll a (Chl a), total phosphorus (TP), dissolved reactive phosphorus (DRP), ionic concentration (measured as electrical conductivity (EC)) and pH. These variables were used to develop diatom-based transfer functions using weighted averaging regression and calibration (simple, tolerance down-weighted and with partial least squares algorithm applied). Overall, models derived for stratified means were weaker than those using annual or isothermal means. For specific variables, the models derived for the isothermal mean of EC (WA-tol r2jack = 0.79; RMSEP = 0.15 log10 S cm–1),the annual mean of pH (WA r2jack = 0.72; RMSEP = 0.25 pH units) and the isothermal mean of Chl a (WA r2jack = 0.71; RMSEP = 0.18 log10 mg m–3 Chl a) performed best. The models derived for TP were weak in comparison (for the annual mean of TP: WA r2jack = 0.50; RMSEP = 0.24 log10 mg m–3 TP) and residuals on estimates for this model were correlated with several other water quality variables, suggesting confounding of species responses to TP concentrations. The model derived for the isothermal mean of DRP was relatively strong (WA-tol r2jack = 0.78; RMSEP = 0.17 log10 mg m–3 DRP); however, residual values for this model were also found to be strongly correlated with several other water quality variables. It is concluded that the poor performance of the TP and DRP transfer functions relative to that of the Chl a model reflects the coexistence of nitrogen and phosphorus limitation within the lakes in the data set. In spite of this, the suite of transfer functions developed from the training set is regarded as a valuable addition to palaeolimnological studies in NewZealand.  相似文献   

18.
A diatom transfer function to infer epilimnetic total phosphorus (TP) concentration was derived using surface sediment diatom data from 68 medium-sized (10–1000 ha) lakes in Southern Finland. Publicly available monitoring records were used in lake selection to avoid gradients caused by pH and humic substances. Constrained and partially constrained ordinations indicated that TP was an important variable influencing diatom assemblages. A long floristic gradient in relation to TP was also apparent and therefore an inference model was developed for TP using unimodal-based regression and calibration methods. The final model included 61 lakes with epilimnetic TP concentrations between 3 and 89 g P l–1, measured during the autumnal circulation period. It has a jackknifed-estimated root mean squared error of prediction of 0.16 log g P l–1, a maximum bias of 0.28 log g P l–1, and an r2 jack of 0.76.The model was tested in the presently eutrophic Lake Valkjärvi (epilimnetic [TP] 60–85 g P l–1), located in Southern Finland. It successfully predicted the measured autumnal epilimnetic TP concentration for the past twenty years and the changes in inferred [TP] reflected disturbances known to have occurred before that time. The diatom-based inferences show that Lake Valkjärvi was oligo-mesotrophic as late as the 1930's and has become eutrophic because of nutrient inputs from agriculture and, especially, municipalities. However, epilimnetic TP concentration has not increased further.  相似文献   

19.
Physical, chemical, and biological data were collected from a suite of 57 lakes that span an elevational gradient of 1360 m (2115 to 3475 m a.s.l.) in the eastern Sierra Nevada, California, USA as part of a multiproxy study aimed at developing transfer functions from which to infer past drought events. Multivariate statistical techniques, including canonical correspondence analysis (CCA), were used to determine the main environmental variables influencing diatom distributions in the study lakes. Lakewater depth, surface-water temperature, salinity, total Kjeldahl nitrogen, and total phosphorus were important variables in explaining variance in the diatom distributions. Weighted-averaging (WA) and weighted-averaging partial least squares (WA-PLS) were used to develop diatom-based surface-water temperature and salinity inference models. The two best diatom-inference models for surface-water temperature were developed using simple WA and inverse deshrinking. One model covered a larger surface-water temperature gradient (13.7 °C) and performed slightly poorer (r2 = 0.72, RMSE = 1.4 °C, RMSEPjack = 2.1 °C) than a second model, which covered a smaller gradient (9.5 °C) and performed slightly better (r2 = 0.89, RMSE = 0.7 °C, RMSEPjack = 1.5 °C). The best diatom-inference model for salinity was developed using WA-PLS with three components (r2 = 0.96, RMSE = 4.06 mg L–1, RMSEPjack = 11.13 mg L–1). These are presently the only diatom-based inference models for surface-water temperature and salinity developed for the southwestern United States. Application of these models to fossil-diatom assemblages preserved in Sierra Nevada lake sediments offers great potential for reconstructing a high-resolution time-series of Holocene and late Pleistocene climate and drought for California.  相似文献   

20.
Aquatic invertebrates are intrinsically capable of rapid and sensitive response to changes in their lacustrine habitat. Fossil invertebrate assemblages preserved in the sediments of a climate-sensitive lake can thus produce high-resolution proxy records of past climate. In shallow lakes, however, a potential conflict exists between the sensitivity of biota to frequent habitat change in their fluctuating environment and the increased probability of disturbance of selected proxy records by bioturbation and physical mixing of sediments. I investigated this problem with tropical-African aquatic invertebrate faunas in a paleolimnological sensitivity study that incorporates both the response of biota to short-term habitat change and the taphonomic integrity of fossil assemblages in a small, shallow, and hydrologically closed lacustrine basin. Analysis of chironomid, cladoceran, and ostracode remains in a210Pb-dated short core from Lake Oloidien (Kenya) indicates that habitat changes accompanying the late 19th- and 20th-century fluctuations in lake level (Z max range: 3 to 18 m) and salinity (conductivity range:c. 400 toc. 2000–4000 µS cm–1) were sufficient to drastically alter the composition of local benthic and planktonic invertebrate faunas. This response remained relatively unaffected by taphonomic phenomena during its incorporation into the sediment record. Results indicate that tropical-African aquatic invertebrate faunas in suitable climate-sensitive lakes are a valuable tool to resolve paleoclimatic fluctuations on a timescale of decades.This paper was presented at the VI Palaeolimnology Symposium, held at Canberra in April 1993. Dr. Mark Brenner guest edited this contribution.  相似文献   

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